草酸与铬酸根形成配位络合物强化可见光还原六价铬的机理研究

The coordination complex formed by oxalic acid and chromate andthe mechanism for enhanced photoreduction of Cr(Ⅵ)undervisible-light irradiation

草酸是自然水体中广泛存在的一种小分子羧酸,主要来源于植物的根系分泌和有机质分解,对自然水体中Cr(Ⅵ)的还原有重要的影响。本研究通过等温滴定量热法(ITC)、电喷雾分辨质谱(ESI-HRMS)分析发现,当pH≤4,草酸根和铬酸根分别以HC2O_4^(-)和HCrO_(4)^(-)的形式存时,可以通过脱水缩合的方式形成具有可见光活性的草酸-铬酸配位络合物(HC_(2)CrO_(7)^(-))。在可光的照射下,HC_(2)CrO_(7)^(-)络合物草酸氧原子上的电子可以通过HOMO-LUMO跃迁的形式向铬酸转移,实现Cr(Ⅵ)的还原。借助电喷雾高分辨质谱检测了Cr(Ⅵ)还原过程中的中间产物,确Cr(Ⅵ)最终还原产物为低毒的Cr(Ⅲ)-草酸络合物([Cr^(3+)-(C_(2)O_4^(2-))_3]^(3-))。根据检测到的反产物,绘制了草酸可见光还原Cr(Ⅵ)的反应路径。本研究将为利用草酸高效光还原Cr(Ⅵ)术提供理论依据。

Oxalic acid,which mainly originates from root exudation and plant residue decomposition,is one of the most widely existiug carboxylic acids,and has a dramatic impact on the reduction of chromate in natural waters.In this study,isothermal titration calorimetry(ITC),theoretical calculations,and electrospray ionization high resolution mass spectrometry(ESI-HRMS)analysis revealed that oxalic acid could complex with chromate to form a Cr(Ⅵ)-oxalate complex via an esterification process at pH≤4.The in situ generated HC_(2)CrO_(7)^(-)complex was a visible-light-active species,and visible light could induce electron transfer from the oxalate unit to the Cr(Ⅵ)within the HC_(2)CrO_(7)^(-)complex via a HOMO-LUMO electronic transition,resulting in Cr(Ⅵ)reduction.Subsequently,we verified the Cr(Ⅵ)reduction intermediates,and the hypotoxic Cr(Ⅲ)species[Cr^(3+)-(C_(2)O_(4)^(2-))3]^(3-)was the final product.Finally,the pathway of Cr(Ⅵ)reduction by oxalic acid under visible-light irradiation was proposed.This study will provide a theoretical basis for efficient photoreduction of Cr(Ⅵ)pollution using oxalic acid.